1. Field of the Invention
This invention relates to a particle detector for use in vacuum or low pressure process equipment and in particular to a particle sensor that is operable in high temperature environments.
2. Description of Prior Art
Semiconductor wafer fabrication processes are particularly susceptible to yield loss due to particle contamination. Free particles landing on a wafer surface during processing can produce defects in patterns and deposit layers that result in device failure. A primary source of these free particles is the vacuum processing equipment that is used for process steps such as the deposition of dielectric and conductive layers, the plasma etching of patterns, electron beam writing of fine patterns, ion implantation of dopants into the wafer surface, and the dry stripping of photoresist. For example, in the process of depositing glass layers on silicon wafers, glass will also deposit in the process chamber. The glass builds up on the walls of the chamber, eventually breaking off and landing on the wafer being processed in the chamber. Similar problems occur in metal deposition systems and other similar types of equipment. Such contamination adversely affects the very fine patterns of the integrated circuits that are being produced. To achieve efficient and high yield semiconductor manufacture, it is essential to determine when the process chamber is dirty from contamination.
U.S. Pat. No. 4,739,177 describes an airborne particle detector that is remote to the semiconductor process while monitoring contaminant particles in an air sample. However, it is known that in many deposition processes cooling of the gases that are used in the various process steps will result in particle generating reactions. When using a remote sensor whereby a sample needs to be drawn, the particle generating reaction places limitations on the use of the sensor.
The aforementioned patent describes a particle sensor for use in vacuum equipment that incorporates a light source, preferably a laser diode, in the optical detection system. When implementing semiconductor processes at high temperature, such as 800.degree. C. by way of example, the performance of the laser diode light source rapidly degrades. Also, the materials conventionally used in semiconductor processes, such as silicon dioxide and silicon nitride, tend to deposit and coat and adversely affect the elements of the optical system which causes a reduction in the sensitivity of the detector system. Furthermore, the relatively long wavelength of the laser diode source that is normally used, which is about 780 nanometers, limits the sensitivity of the detector system, since light scattering intensity scales as the ratio of particle diameter to wavelength.
Also an additional problem experienced with using laser diodes in high temperature environments is the occurrence of very intense background radiation having a wavelength close to that of the laser diode. The background radiation is generated because the furnace used in the semiconductor manufacturing process acts as a black body source. The higher the temperature of operation, the shorter the wavelength distribution of the radiation will be.
FIG. 1 illustrates the intensity of background radiation due to black body radiation in the bandwidth of 750 to 850 nanometers as a function of temperature. The sensor described in U.S. Pat. No. 4,739,177 typically requires about 100 nanowatts per square centimeter of scattered radiation to sense an 0.5 micron diameter particle. The background radiation exceeds this level of intensity at slightly above 400.degree. C. At about 800.degree. C., the sensor disclosed in the aforementioned patent would saturate. It is apparent that it is difficult to provide a detector that is sensitive to very small particles in the temperature range between 400.degree. C. and 800.degree. C. and above when using a laser diode source that typically emits at 780 nanometers.